1. Field of the Invention
The present invention relates in general to a robot arm of a pantograph type, and more particularly to a technique for stabilizing the direction of movement and the operating posture of a movable member attached to the free end of a linkage of the robot arm.
2. Discussion of the Prior Art
In the art of industrial robots, there is known a pantograph type, expandable robot arm which has a plurality of rigid link bars linked together in a parallelogram form, and which is expaned and contracted in a direction along a straight line. An example of such a pantograph type robot arm is disclosed in laid-open Publication No. 60-6369 of unexamined Japanese Patent Application. The parallelogram linkage of the robot arm carries a movable member pivotally attached to its distal or free end for effecting a desired operation such as a welding operation. While the parallelogram linkage is pivotally supported at its proximal or fixed end by a suitable stationary support member or structure, the operating posture of the movable member during movement thereof due to expansion or contraction of the linkage is not sufficiently stable. Accordingly, the path of movement taken by the movable member may more or less deviate from the nominal straight path. Further, the pivotal connection of the movable member to the distal link bars of the parallelogram linkage tends to lead to instability of maintaining the movable member in its predetermined posture, which may result in positioning accuracy of the movable member that is not acceptable in some applications of the robot.
In the light of the above, it is considered possible to provide suitable guiding means for guiding the movable member along the desired nominal straight path, while maintaining the predetermined posture. This arrangement is necessarily complicated, and suffers from an interference of the guide means with the other members when the parallelogram linkage is contracted, with a guide member or members of the guiding means projecting beyond the support structure. Accordingly, the operation of the robot or the operating range or area of the robot arm or movable member is more or less limited.